In this study, a nano‐focus X‐ray computed tomography (X‐CT) is used to observe the formation of methane hydrate in sands on pore scale and to quantify hydrate saturation, pore structure parameters, and permeability of hydrate‐bearing sands. A new analytical technique is developed to improve the identification of water‐hydrate boundary. Three hydrate accumulation habits (i.e., floating, contacting, and cementing) in pore spaces are observed. When there is no methane bubble, hydrate distribution evolves from floating to contacting and then to cementing as hydrate saturation increases. In contrast, only contacting and cementing distribution patterns appear in the pores with methane bubbles. Based on the surface extraction and defect detection from 3‐D images, both the gas/water/hydrate distributions and the pore structure characteristics are investigated. The results show that the porosity, the maximal pore volume, and maximal diameter decrease as hydrates accumulate in the pores. However, the maximal pore surface, the total pore number, and the large pores (>100 voxels) number increase gradually until the hydrate saturation reaches 35%, when they turn to decrease rapidly. The pore structure parameters show detailed changes of hydrate and water on pore scale. The incompressible Navier‐Stokes equations are used to calculate absolute permeability of hydrate‐bearing sands based on X‐ray computed tomography digital images. It indicates that the microdistribution of the hydrate has a significant effect on the permeability calculation. The fluid streamlines obtained through simulation are used to track the water transport paths.

The characteristics of the hydrate-bearing layer are important parameters for gas hydrate exploration and exploitation, which can be provided by laboratory analysis. We have carried out a systematic analysis of modern instruments, including the laser grain size analyzer, X-ray computed tomography, scanning electron microscopy, X-ray diffraction (XRD), Raman spectroscopy, gas chromatography, and isotope mass spectrometry, on the hydrate-bearing sediment samples recovered from the Shenhu area in the South China Sea in 2016. We focused on the structure, hydration number, and gas composition of the gas hydrate samples. We also evaluated the effects of sediments on the occurrence and distribution of gas hydrate. Our results showed that the sediments are much finer than those recovered from the Shenhu area in 2007. Some samples are found to contain calcareous microfossils and foraminifera, which are beneficial for dispersed gas hydrate formation. Whereas the only visible hydrate sample (FC4) has not found any bioclast in the sediments. Based on the Raman data and the XRD analysis, the gas hydrates demonstrate typical structure I (sI) hydrates, with cage occupancies of methane molecules higher than 99.3% in large cages and 81.5%–91.4% in small cages, respectively. The crystal lattice parameter is 11.89 Å with the hydration number being approximately 6.0. Methane is obviously the dominant component, ranging from 97.6% to 99.95% in these samples. Comprehensive study of carbon and hydrogen isotopes suggests that the hydrate-bound gases are from biogenic and thermogenic gases in the study area.

Geophysical detection techniques are important methods in marine gas hydrate exploration and monitoring, because the small-scale distribution of hydrates has a large impact on the wave velocity. The acoustic response characteristics of hydrate micro-distributions have strong significance for monitoring the hydrate dissociation process. In this paper, experiments simulating the hydrate dissociation process were carried out in a self-developed experimental device combining X-ray computed tomography (X-CT) scanning and ultrasonic detection, which allowed the acoustic wave characteristics and X-CT scanning results to be simultaneously obtained during the hydrate dissociation process. This study found that the hydrate dissociation stage is divided into three stages. The hydrate begins to dissociate at spots where it comes into touch with sand particles early in the dissociation process. The main factor affecting the acoustic wave velocity of hydrates in this stage is changes in the microscopic distribution of hydrate. In the middle stage, a large amount of hydrate decomposes, and the main factor affecting the acoustic wave velocity of hydrate in this stage is the change in hydrate content. In the later stage of hydrate dissociation, the hydrate distribution pattern consists mainly of the pore-filling type, and the hydrate micro-distribution at this stage is the main factor affecting the acoustic wave velocity. This study will be of great significance for understanding the microscopic control mechanism of hydrate reservoir geophysical exploration.

Natural gas hydrate is widely spread in marine environments around the world. It has great energy potential due to its high methane gas content. High-precision exploration and evaluation of marine gas hydrate still face great challenges as it is affected by the complex reservoir control mechanisms and distribution characteristics. Resistivity is widely used in geophysical logging and theoretical research on gas hydrate-bearing reservoirs by utilizing the high sensitivity electrical response. In this paper, based on the examination of the global marine gas hydrate occurrences, resistivity logging results are summarized. Then the key remaining gas hydrate resistivity experimental concerns are reviewed. In summary, resistivity properties are a reliable means to derive the gas hydrate reservoir characteristics, despite the effect induced by the anisotropic properties of hydrate reservoirs and drilling technology. The overall resistivity change associated with the occurrence of pore filling gas hydrate in reservoirs are relatively small, and the specific value is affected by sediment lithology and hydrate saturation. On the other hand, fracture filling hydrate reservoirs have strong anisotropy, and massive hydrate occurrences (i.e., layers of gas hydrate with no sediment) section shows very high resistivity variation. Clay minerals are an important factor restricting the accurate estimation of gas hydrate saturations from in situ resistivity measurements. Many experimental studies have proposed the correction of Archie empirical formula, but widely representative models have not yet been developed. It is worth noting that more complex resistivity measurements may be able to provide additional electrical response information on various gas hydrate systems.